nRF51822 carrier-only test mode

[florentb]

I am testing the DTM mode on the nRF51822_QFAA, and I use the carrier-only test with the following command :

dtm_init();    
dtm_cmd(LE_TRANSMITTER_TEST, 39, CARRIER_TEST, DTM_PKT_VENDORSPECIFIC);
while(1);

The result, with a spectrum analyzer (radiated test), looks like this:

While this seems within spec, is it normal to have the two additionnal peaks, 165kHz apart from the carrier?

Or is it caused by a PCB / design issue?

I am sure these are not coming from my spectrum analyzer, as I reproduced it with several analyzers and also with several nRF devices with different antennas.

[florentb]

Yes, I tested it on several custom boards. It's not the power supply (same results with a battery + LDO).

What I found out is that CPU activity has a strong influence on the spectral purity of the carrier. Depending on the instructions that you execute after entering test-mode, you will get different results - from a clean carrier (sleep mode), to a heavily modulated carrier (if you execute various code). Sometimes you will get only few peaks (as previously).

Using code in SDK10\examples\dtm\direct_test_mode\main.c, the results are very bad, as you execute a lot of various code after entering constant-carrier mode. Can you try reproduce that on a Nordic dev board?